Radio units, and especially small radio units for mobile radio communication, are occasionally equipped with small antennas. This means that the center of radiation and the most powerful field of radiation from the antenna and the housing of the radio unit is situated close to the user's ear. In order to get around this problem, it is desirable to lift off the center of radiation some distance from the user's ear.
It is previously known that if a radio unit is supplied with a half-wave dipole antenna, the axis of the field of radiation lies in the middle of the antenna. Consequently, by making the antenna sufficiently long, the field will be moved away from the ear and the intensity of radiation will be considerably reduced near the user's ear and head.
Drawbacks associated with half-wave dipoles and other types of high-impedance fed dipoles are that they are difficult to match in impedance, especially if the intention is that the antenna should cover two or more harmonic bands. If, for example, a half-wave dipole is fed at one of its ends, it requires very high feeding impedances, in the magnitude of 800 ohms. If the dipole is fed at its center part, the feeding impedance is considerably lower, in the magnitude of 70 ohms. With small mobile radio units the antenna is as a rule fed from one of its ends. Simultaneously, the power stage, which is feeding the antenna, is provided with a much lower output impedance, in the magnitude of 50 ohms. To prevent the occurrence of reflections and a low degree of efficiency, the low output impedance of the power stage is to be matched to the high feeding impedance of the antenna. This requires an impedance-matching device be coupled up between the antenna and the power stage. The impedance-matching device may also be called a impedance-adapting device, or shorter, impedance-adapting, impedance-matching or just matching.
Different types of impedance-matching devices are previously known. One previously known type of matching constitutes a transformer with resonant circuits. In principle, a primary part is associated with the output of the power stage and a secondary part, comprising the tuned resonant circuits, with the antenna. The resonant circuits contain a parallel coil and a capacitance. The coil may occasionally be provided with an air core. In one variation of resonant circuits, the core is formed by means of a strip line, which means that a printed board pattern is produced to form the coil. In another variation, the primary winding is omitted and the conductors from the power stage are directly connected to any suitable position on the secondary winding. This solution involves advantages, such as fewer and smaller components which saves space and costs compared to a transformer circuit with both primary and secondary windings. One considerable drawback associated with this solution is its narrow bandwidth.
A further type of impedance-matching involves the use of a helix resonator, which in fact is a filter component, which in extreme cases may function as a tuned oscillation circuit.
In small apparatuses, as for example mobile radio equipment, however, only a small space is offered for an impedance-matching device.